The overall goal of this project is to elucidate the role of iron acquisition by Mycobacterium avium complex (MAC) organisms in the intracellular survival of these mycobacteria. MAC are an important cause of morbidity and mortality in patients with AIDS. The hypothesis is that host iron-binding proteins, transferrin and lactoferrin, may facilitate iron delivery to intracellular MAC. The Specific Aims of this proposal are: (1) to examine the influence of iron and host iron-binding proteins on growth, uptake and intracellular survival of MAC; (2) to study the regulation of transferrin receptor expression on MAC-infected cells; and (3) to elucidate the mechanisms of iron incorporation into intracellular MAC. The investigator will employ a radiometric growth assay for MAC, and a murine peritoneal macrophage-like cell line (J774A.1) will be utilized.